This invention relates to a hydraulic brake system for automotive vehicles which is provided with a hydraulic power booster and a master cylinder operable by the power booster. The booster piston is designed as a stepped piston with a pedal-close portion of smaller diameter and a pedal-remote portion of larger diameter. Together with a housing, the annular surface of the booster piston at the piston step encloses an annular space of variable volume. The annular space communicates with an unpressurized supply reservoir, which comunication can be locked, wherein in a pedal-remote pocket bore of the booster piston a master cylinder piston is guided in a sealed manner which confines a working chamber. An annular chamber confined by the pedal-remote annular surface of the booster piston is pressurized by an auxiliary pressure source with a hydraulic communication being established between the annular chamber and the working chamber by way of a non-return valve which opens towards the working chamber.
The above characteristics are generally described in German patent application No. P3328104.1. In the brake system described more fully herein, a booster piston is provided whose pedal-close effective surface is pressurized by a pressure which is proportional to the respective actuating force exerted on the brake pedal. At the pedal-remote end surface of the booster piston a stepped piston is supported whose pedal-remote end surface has a pocket bore. In the pocket bore a master cylinder piston is slidingly guided while being sealed. The stepped piston has a pedal-close portion of smaller diameter and a pedal-remote portion of larger diameter.
Between the piston portions an annular surface results which, in coordination with the housing, confines a circumferential annular space communicating with an unpressurized supply reservoir by way of a shut-off valve. The pedal-remote annular surface of the stepped piston confines a circumferential annual space communicating with this space by way of a throttle device. Moreover, by way of a further valve device, there exists a communication between the circumferential annular space confined by the pedal-remote end surface of the stepped piston and the unpressurized supply reservoir--which communication may be locked. The further valve device is designed as double-seat valve and locks communication towards the unpressurized supply reservoir upon appropriate pressurization.
Upon pressurization of the additional valve device, and simultaneously with the locking action towards the unpressurized reservoir, there is established a hydraulic communication between a pressure source and the circumferential annular space confined by the pedal-remote annular surface of the stepped piston. The result of such pressurization of the circumferential annual space is that pressure medium which, if necessary, was tapped from the working chamber will be filled up by way of a gasket arranged between the stepped piston and the master cylinder piston, thus a minimum volume reserve of the working chamber is ensured.
In the brake system described in the previously identified German patent application provision is made for communication between the annular space confined by the pedal-remote end surface of the stepped piston and the annular space confined by the piston step and which may be locked in dependence on the travel of the stepped piston so that a limitation of the pedal stroke as a function of the travel covered by the stepped piston results.
In certain applications it is advantageous for an automotive brake system to have a so-called "prefill effect" which means that, at the beginning of the braking operation, a relatively large piston surface is effective temporarily, and, thus the required absorption volume is supplied relatively fast in the initial phase of braking. Such a measure is not provided in the brake system described in the above mentioned patent application. It is therefore an object of the present invention to further develop a brake system of the type described so as to ensure that the required absorption volume is introduced into the brake circuits at the beginning of a braking operation.